1. Technical Field
This invention relates generally to maximizing radio frequency transmission power in a wireless communication device transmitter, and, more particularly, to a system for a dual feedback translation loop for power amplifier feedback control.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes where various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, portable communications systems complying with the IS-136 standard specify the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset xcfx80/4 differential quadrature phase shift keying (xcfx80/4-DQPSK) or variations and the access format is time division multiple access (TDMA). Other standards may require the use of, for example, code division multiple access (CDMA).
Similarly, in Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrowband TDMA access environment. In a typical GSM mobile communication system using narrowband TDMA technology, a GMSK modulation scheme supplies a very clean phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient, non-linear power amplifier can be used, thus allowing efficient transmission of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Other transmission standards, such as that employed in IS-136, however, use a modulation scheme in which the transmitted signal is both phase modulated (PM) and amplitude modulated (AM). Standards such as these increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, existing GSM radio transmitters are not easily adapted to transmit a signal that includes both a PM component and an AM component. One reason for this difficulty is that in order to transmit a signal containing a PM component and an AM component, a highly linear power amplifier is required. Unfortunately, highly linear power amplifiers are very inefficient, thus consuming significantly more power than a non-linear power amplifier and drastically reducing the life of the battery or other power source.
This condition is further complicated because transmitters typically employed in GSM communication systems transmit in bursts and must be able to control the ramp-up of the transmit power as well as have a high degree of control over the output power level over a wide power range. In GSM this power control is typically performed using a closed feedback loop in which a portion of the signal output from the power amplifier is compared with a reference signal and the resulting error signal is fed back to the input of the power amplifier.
When attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will tend to fight against the amplitude variations present in the signal while attempting to maintain the desired output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal.
In such systems in which transmit signals contain both PM and AM components, the output power can be controlled by setting a calibrated control signal on the power amplifier. Unfortunately, this requires the use of a highly linear, and therefore very inefficient, power amplifier. In non-burst transmission systems the output power may be controlled by a feedback loop having a time-constant that is very low compared to the time-constant of the amplitude variations of the modulator. Another known method to control the output power is to xe2x80x9cpre-distortxe2x80x9d the modulated signal in such a way that the power control loop will cancel the effect of the pre-distortion. In such a method, the amplitude information is passed through a transfer function that is the inverse of the power control loop transfer function. Unfortunately, these methods are costly and inefficient.
In those transmission standards in which both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier. Indeed, even in systems in which only a phase modulated component is amplified, phase distortion in the power amplifier may degrade the phase modulated signal.
With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to allow portable transceivers to transmit a signal containing both a PM component and an AM component, while maximizing the efficiency of the power amplifier. Furthermore, as the GSM standard evolves further, such as with the development of enhanced data rates for GSM evolution (EDGE), it is desirable to have one portable transceiver that may operate in all systems. Further still, in systems in which only a phase modulated signal is transmitted, it would be desirable to eliminate any phase distortion caused by the power amplifier.
The invention provides a system for a dual feedback translation loop for power amplifier feedback control that maximizes power amplifier efficiency.
The invention provides a dual feedback translation loop for controlling a power amplifier in which during a first time period the output of the translation loop is used as phase control feedback to the translation loop and during a second time period the output of the power amplifier is used as feedback to the translation loop. The feedback from the translation loop is supplied to a first phase detector and the output of the power amplifier is supplied to a second phase detector. Each phase detector also receives as input a modulated transmit signal. The phase difference between the modulated transmit signal and the respective feedback signals are measured by the phase detectors and used to drive first and second charge pumps, respectively coupled to the output of the first and second phase detectors. The charge pumps are configured to operate such that the sum of the currents during the phase correction pulses from the two charge pumps is constant, thus providing a smooth switching function between the output of the translation loop and the output of the power amplifier supplied as feedback to the translation loop. During transition from one charge pump to the other, the current pulses from the first charge pump become weaker, while the current pulses from the second charge pump become stronger until the transition is complete.
Related methods of operation and computer readable media are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.